1. Field of the Invention
The present invention relates to an image forming device wherein both a monochromatic image and a full color image can be printed and in addition, the editing, the compression and the expansion of an image for every small area can be repeated, particularly relates to an image forming device wherein the print speed of monochromatic image data is enhanced at a low cost.
2. Description of the Related Art
For a device that forms a color image at high speed, as shown in FIG. 4, there is one wherein paper is carried using a printing head including a recorder dedicated to each of YMCK and each recorder is operated according to pipeline at the speed of a process. In such configuration, a controller for supplying an image is required to supply 4-color image data to each recorder at the speed of a process. As the controller is required to supply image data asynchronously passed from an image generator at the speed of a process, a general method is one with steps of once compressing and storing an image and supplying its image data at the speed of a process decompressing it.
In such an image forming device (a full color page printer), processing is normally executed as follows. As shown in FIG. 4, (1) an image generator 1 first interprets an input file described in page description language (PDL) and others and generates an image. (2) An image processor 2 executes image editing processing including turning, scaling and substituting, color conversion (from RGB to YMCK) and others. (3) An image compressor 3 provided for every color component compresses and encodes an image for every color component. (4) An image storage 4 stores a compressed image for every color component. (5) An image decompressor 5 provided for every color component decompresses the corresponding compressed image. (6) A print head 7 prints the decompressed image.
Normally, in image processing shown in (2) such as image editing processing and color conversion, a page memory is required at least for every color component. As the page memory increases the cost, a method of using the image storage 4 in place of the page memory used by the image processor 2 is proposed. In this case, as image data to be processed is stored in the image storage 4 with it compressed, the compressed image is decompressed, is fed back to the image processor 2 as shown by a broken line in FIG. 4, image processing is applied to the decompressed image, afterward, the decompressed image is compressed again and is written in image storage 4. That is, the image compressor 3 and the image decompressor 5 are required to be used together, executing image editing processing and others.
For an example of this method in prior art, technique disclosed in Japanese Published Unexamined Patent Application No. Hei 5-31974 will be described. In the above example of prior art, a macroblock (hereinafter called MB) image acquired by dividing an image into small rectangular areas is adopted as an edited object. Editing processing is applied to the MB image for every image editing command described in PDL, and compression and storage are repeated. In the case of overlap with an MB image to be a background in substitute processing and others, the data of the corresponding MB image is generated by decompressing stored compressed data referring to an MB management table. FIG. 5 schematically shows the above processing and in this example, the respective images of YMCK are treated in units of MB image labeled as (1) to (6), are compressed in units of MB image by the image compressor 3 and are stored in the image storage 4. The decompressor 5 decompresses the image in units of MB image. If image processing such as substitute is required, the corresponding MB image is decompressed and is fed back to the image processor 2.
If the image processing in above (2) and the compression of an image in above (3) are executed without dividing an image into MB images in this method, the cost required for a page memory is reduced, however, if an image in a slight area is overwritten, all images to be a background are also required to be decompressed and the processing speed is reduced. Therefore, if the image processing in above (2) and the compression processing in above (3) are executed in units of MB image in a page printer, the reduction of the cost and the processing speed can be balanced.
If a black-and-white binary image or an only black monochromatic multivalued image is printed in the above printer, the same printing speed as that of a color image is realized using only an image processing path for K. However, as improvement for the resolution of a binary image or a monochromatic multivalued image (hereinafter these will be called a monochromatic image) is more and more demanded, the information capacity of these becomes equal to that of a color image. For example, for the information capacity of a color image on a full raster in A4 size according to JIS, if the resolution is 600 dots/25.4 mm and the gradation precision is 8 bits/pixel, the data amount for one color is approximately 35 MB.
In the meantime, for the information capacity of a binary image in the same size, if the resolution is 9600 dots/25.4 mm and the gradation precision is 2 bits/pixel, the data amount for one color is approximately 70 MB. Also, for a monochromatic multivalued image, if the resolution is 1200 dots/25.4 mm and the gradation precision is 8 bits/pixel, the data amount for one color is similarly approximately 70 MB and they have double information capacity, compared with the information capacity for one color of a color image.
If the compression and expansion of an image are implemented by hardware such as ASIC, the number of pixels processed per unit time (throughput) is fixed because of the character of a pipeline implemented by ASIC. Therefore, in such a printer according to a tandem system, if a monochromatic image having double information capacity compared with that of a color image is printed, image data cannot be supplied unless process speed at which paper is carried is halved and blank paper or paper printed halfway is output.
In an example of prior art, the reduction of a cost and high processing speed are balanced by executing image processing in above (2) and compression processing in above (3) for every MB in a full color page printer.
However, for a monochromatic image, there is a problem that the load of compression processing in above (3) and expansion processing in above (5) is large, hardware dedicated to a monochromatic image is required to be added to enhance processing speed and the cost and processing speed cannot be balanced.
The present invention has been made to solve the above problems and provides a method of printing a monochromatic image at faster speed than that of a color image without greatly increasing the cost and balancing the low cost and high processing speed in a full color printer according to a tandem system.
First, referring to FIG. 1, the present invention will be roughly described below.
In an image forming device according to the present invention, if a monochromatic image is printed, an MB image divided by an image processor is sequentially sent to the respective image compressors for YMCK. Four image compressors are operated in parallel and the compressed data of an MB image is stored in an image storage. After the data of all MB images required for an output image is stored, an MB image controller operates the respective image decompressors for YMCK in parallel and sends the stored data to the image decompressors. The decompressed data of MB images spatially divided is coupled to one image data by an image coupler and is sent to a print head.
The MB image controller can enhance process speed at which paper is carried in the print head by amount in which the processing speed of a monochromatic image is enhanced according to a situation in which an MB image is decoded.
The present invention will be further detailedly described. According to the present invention, an image forming device which can print a full color image is provided with: an image processor for converting an image to a final printout image in units of small area; plural image compressors for receiving the image in units of small unit from the above image processor and compressing it; an image storage for storing the compressed image transferred from the above image compressor; plural image decompressors for decompressing the compressed image stored in the above image storage in units of small area; a temporary image storage for temporarily storing the image decompressed by the above image decompressor; a print head for printing the image stored in the above image buffer by the corresponding recorder for every component; and a small area image transfer controller for allocating and transferring the image in the units of small area transferred from the above image processor to the plural image compressors according to whether the image is monochromatic or a full color and allocating and transferring the compressed image stored in the above image storage to the plural image decompressors in the units of small area. The above small area image transfer controller controls the transfer of an image in the units of small area by compressing an image in units of small area for every color component by the image compressor corresponding to the color component and further, decompressing it by the image decompressor corresponding to the color component in case an image to be printed is a full color and compressing a monochromatic image in units of small area by the respective plural image compressors in parallel and further, decompressing it by the plural image decompressors in parallel in case an image to be printed is monochromatic.
In the above configuration, as resources for compressing and decompressing in colors other than the following color component (not normally used for printing a monochromatic image) are used for compressing and decompressing the monochromatic image when the monochromatic image is printed, high speed processing can be implemented at a low cost by the amount. Even if an image to be printed is a monochromatic image having high resolution, it can be printed at speed equal to or faster than a normal full color image. The above monochromatic image includes a binary image and a multivalued image (a gray scale).